Patent Application
20060066328
The present invention relates to the field of electrical testing devices for microchips, and more particularly to a hybrid-bucking beam probe test assembly for contacting a footprint of a chip.
Semiconductor chips are connected to external electronics through contact pads manufactured on the semiconductor chip. Wire bonding and flip-chip bonding are two of the most common methods of forming electrical connections between semiconductor chips and external electronics. In wire bonding, a plurality of bonding pads are located in a pattern on the top surface of the substrate. This pattern of bonding pads is referred to as the substrate's footprint. Fine wires, typically made from gold or aluminum, are connected between the contacts on the substrate and bonding pads formed in a microchip packaging. Flip-chip bonding is an efficient method of electrically coupling a chip to external electronics. In the flip-chip technique, the top surface of the semiconductor chip has an array of electrical contact pads. A solder bump is formed on each of the contact pads. The chip packaging has a corresponding grid or array of contact pads. The chip is flipped upside down so that the solder bumps on the chip mate with the grid of contact pads in the package, hence the name “flip-chip.” The assembly is heated to flow the solder plaiting on the chip contacts. As with wire bonding, the pattern of solder bump contacts on the chip is referred to as the footprint. With this array/grid of solder bumps, these chips are often called area-array solder-bump devices.
The profitability of microchip manufacturers is dependent upon the ability to test and probe semiconductor chips for quality assurance. Semiconductor chips are sometimes defective and it is undesirable for economic reasons to package defective chips as packaging often represents an expensive step in the fabrication of integrated circuits. Consequently, it is highly desirable to test semiconductor chips before they are packaged. In addition, testing semiconductor chips enables companies to maintain the reliability and quality in fabrication processes. Tesing the chips prior to packaging is often referred to as wafer probing. Wafer probing also enables manufacturers to work toward increasing the yield of its fabrication line, thereby improving profit margins.
The process of wafer probing is such that probes are used to establish electrical contact with the pads formed on the semiconductor chip. The probes are used to apply test voltages at the pads for testing the response of the semiconductor chip to determine whether it is defective. Semiconductor chips that pass the test are packaged and defective semiconductor chips are discarded.
Current trends in the microelectronic industry portend ever increasing chip densities, which translate in the need for new probing devices that can accommodate the increased number of contact pads formed on the chip.
Probing integrated circuits in the early days of the industry consisted of contacting a relatively small number of points on a chip. This fact is the case even today in applications such as in-line testing where resistance measurements are commonly made using 4-point probes. In general, however, the trend has been toward simultaneously contacting more and more points.
Testing of chips has placed increasing demands on testers and probe hardware. With linear and peripheral footprints it is possible to utilize commercially available contactors. Cantilever contacts are commonly used and well known for peripheral-type footprints. However, the advent of area array or matrix footprints has required the development of probing devices that can accommodate virtually any footprint.
Cast channel probes are one type of testing device for contacting a chip footprint. With a cast channel probe, remote miniature coil springs activate contact wires contained in fine tubing. However, this probe-type has high frequency and footprint limitations. Buckling beam probes using vertical wire columns is another type of testing device for contacting the footprint of an area array device. Buckling beam probes employ the principal of a buckling column, whereby the application of a force beyond the critical load causes buckling to occur. Lengths of 600 to 700 mils (0.6 to 0.7 inches, or 15 to 18 millimeters) are common for buckling beam probes. This long length of the buckling beam probes produces the electrical problem of a resultant inductance as well as signal crosstalk. Forming probes with a hybrid-buckling beam greatly reduces these electrical problems experienced with longer buckling beam probes. These hybrid-buckling beams are flattened and precurved, thereby allowing for a shorter probe length, thus reducing inductance and signal crosstalk. Hybrid-buckling beam probes are also known as COBRA probes, which is a registered trademark of Wentworth Laboratories, Inc., due to their cobra like shape.
Current hybrid-buckling beam probe assemblies suffer from many drawbacks. Fabricating a probe assembly commonly requires troublesome and problematic processes whereby the probes are glued to the probe assembly. In addition, the repair and replacement of damaged or worn probes typically requires the complete disassembly and reassembly of the entire probe assembly. There is therefore a need to improve the design of hybrid-buckling probe assemblies to address these problems and inter alia, improve the quality, function, cost, and manufacturability of hybrid-buckling beam probe assemblies.
The present invention is an improved hybrid-buckling beam probe assembly for probing semiconductor chips. The probe assembly includes an upper and lower die. A template is attached to a boss on the lower die. This template improves the reliability, time, and cost of assembling the hybrid-buckling beam probe assembly. In addition, the template facilitates on site repair and replacement of hybrid buckling beam probes that become damaged or worn during use. An optional spacer may be attached between the upper and lower dies. A template alignment tool is used to attach the template to the boss.
Referring to the Figures by characters of reference,
An upper portion of hybrid-buckling beam probes 36 extend through micro-holes 32 formed in template 26 up into micro-holes 30 formed in upper die 22. Hybrid-buckling beam probes 36 protrude from the top surface of upper die 22 through micro-holes 30. A lower portion of hybrid buckling beam probes 36 extend down through micro-holes 34 such that they protrude from the bottom surface of lower die 28.
Template 26 is attached to lower die 28 in one exemplary method with adhesive strips 38. Other methods of attaching template 26 to lower die 28 include ultrasonic welding, adhesive glue, mechanical fasteners, or other conventional means of attachment known to the art. Lower die 28 has a boss 40 formed therein. Boss 40 is a raised rectangular area that defines a rectangular cavity 42. The rectangular shape of boss 40 is merely exemplary and boss 40 may be formed in other geometric configurations that are capable of providing a raised surface for supporting template 26. While boss 40 is illustrated as being molded into lower die 28, boss 40 may be a separate component that is attached to lower die 28. Boss 40 functions as a raised surface to support template 26. Template 26 is attached to the top surface of boss 40 in one exemplary method with adhesive strips 38. Adhesive strips 38 extend partially over both the top surfaces of boss 40 and template 26, thereby attaching template 26 to boss 40. When template 26 is attached to boss 40, rectangular cavity 42 becomes an enclosed chamber with hybrid-buckling beam probes 36 protruding therefrom. Hybrid-buckling beam probes 36 are held in relative position within rectangular cavity 42 by template 26 and lower die 28. Hybrid-buckling beam probes 36 are made of a conductive material. Hybrid-buckling beam probes 36 may be formed from wire or etched from a sheet of conductive material. The wire or conductive material may be copper, gold, gold-plated copper, copper-plated steel, gold-plated steel, or the like. Other exemplary materials for the probes are PALINEY 7, which is a registered trademark of the J.M. Ney Company, or BeCu. In a nanotechnology implementation, probes 36 could be nanotubes. MEMS is another implementation where the probes are formed by electro-plating methodologies.
Spacer 24 rests directly against the top surface of lower die 28 when probe assembly 20 is assembled. Spacer 24 is provided with a rectangular opening 44 that allows boss 40 to protrude up through spacer 24.
Hybrid-buckling beam probe assembly 20 is used for probing semiconductor chips 70, as shown in
Screws 46 attach probe assembly 20 to printed circuit board 66. Screws 48 attach upper die 22 and spacer 24 together. Screws 49 attach lower die 28 and spacer 26 together. Holes 50 are provided in upper die 22, lower die 28, and spacer 24 for screws 46 to attach probe assembly 20 to the printed circuit board 66. Holes 52 are provided in upper die 22 and spacer 24 for screws 48 to attach upper die 22 to spacer 24. Holes 53 are provided in lower die 28 and spacer 24 for screws 49 to attach lower die 22 to spacer 24.
Hybrid-buckling beam probe assembly 20 is used repeatedly to test numerous semiconductor chips. During this testing, probe assembly 20 may require cleaning or repair. It therefore becomes desirable to have the ability to remove probe assembly 20 from printed circuit board 66. It is also desirable to have the ability to disassemble probe assembly 20 in order to gain access to damaged or worn probes 36 for replacement. Machine screws 46 are used in order to allow for the repeated removal and reattachment of probe assembly 22 to the printed circuit board 66. Machine screws 48 and 49 allow for the repeated disassembly and reassembly of probe assembly 20 for maintenance.
Gauge pins 80, illustrated in
A preferred material for template 26 is a polyimide, such as KAPTON, which is a registered trademark of DuPont. Other various plastics familiar to those in the art may be used to form template 26, such as polymethyl methacrylate (commonly known as LUCITE or PLEXIGLAS), polystyrene, NYLON, or polyethylene. A preferred material for adhesive strips 38 is KAPTON tape. However, other adhesive strips know to the art for attaching KAPTON or various other plastics to the top surface of boss 40 may be used. Other methods of attaching template 26 to lower die 28 include ultrasonic welding, adhesive glue, mechanical fasteners, or other conventional means of attachment known to the art.
The solder bumps 68 in semiconductor chip 70 will have slight variations in height. Probes 36 are able to buckle and bend allowing contact head 60/61 to move perpendicular to the plane of the semiconductor chip 70 and solder bumps 68 on it. Probes 36 are placed into contact with the solder bumps 68 by aligning probes 36 with the footprint of chip 70. Probe assembly 20 is then pressed down against the footprint of chip 70. Probes 36 that come into contact with taller solder bumps 68 will place an upward force against those probes 36 causing them to buckle and move upward, thereby allow the other probes 36 to come into contact with the shorter solder bumps 68. Thus, probe 20 accommodates height discrepancies between solder bumps 68.
Hybrid-buckling beam probes 36 acquired this name due to their mechanical behavior. Probe assembly 20 functions upon the basic buckling beam principle of mechanics. This principle is based on mathematical equations that define the bending, or buckling, of columns as a function of loads placed on them and the geometry of the columns. These relationships, when applied to buckling beams, show that when a force is placed on contact head 60/61 of probe 36, that probe 36 will bend and generate a spring force that is generally independent of the amount of vertical displacement of contact head 60/61. As a result, it is possible to press an array of probes 36 against the footprint of chip 70 such that all probes 36 contact all solder bumps 68 with a relatively uniform amount of force between each probe 36 and solder bump 68, thereby enabling probe assembly 20 to be used to perform quality tests upon semiconductor chip 70.
Probe heads 56 extend through micro-holes 30 and protrude above the top surface of upper die 22. Contact heads 60/61 then contact solder bumps 68 formed on the printed circuit board 66. Normal electrical current flowing through chips 70, which are connected to printed circuit board 66 via solder bumps 68, provides test current to probe assembly 20 for the testing of semiconductor chips 70. Probe heads 56 also extend through micro-holes 32 formed in template 26. Contact heads 60/61 of probes 36 extend through micro-holes 34 formed in lower die 28 and protrude below the bottom surface of lower die 28. Precurved beams 58 of probes 36 are able to buckle or bend within cavity 42, thereby allowing contact heads 60/61 to move vertically within micro-holes 34. In addition, each probe 36 is capable of independently buckling, thereby allowing each contact head to have a different amount of vertical displacement as probe assembly 20 is placed in contact with the area array of solder bumps. As a result, probe assembly 20 can form contact between all probes 36 and all solder bumps 68 in the footprint when the solder bumps 68 are not uniformly flat.
It is desirable to form micro-holes 36 in the region of upper die 22 which has recess 62, due to the fact that the thickness of upper die 22 is less within recess 62 than in the remainder of upper die 22. Having this thinner surface within recess 62 improves the ability to manufacture micro-holes 30 and assemble probes 36 with upper die 22. Micro-holes 34 are also formed in a thinned surface of lower die 28. Cavity 42 formed within boss 40 is created such that the bottom surface is thinner than the remainder of lower die 28 in order to support micro-holes 34. The thinned central bottom portion of lower die 22 within cavity 42 has a thickness less than the outer portions of lower die 22. It is desirable to have micro-holes 34 formed in the thinned surface of lower die 22 to help facilitate the movement of contacts 58 as probes 36 buckle, bend, and move when probing a solder bump device. The thinned surface of lower die 28 within cavity 42 also aids in the manufacture and assembly of microholes 34 with probes 36. These microholes may be laser drilled, machine drilled, or photoetched.
In step 108, template 26 is attached to a template alignment tool 72, illustrated in
Preparation for the attachment of upper die 22 is made in step 114. First, template alignment tool 72 is unclamped from lower die 28. Template alignment tool 72 is then rotated to determine if all of probes 36 loaded into probe assembly 20 can move freely. Probes 36 that do not move freely are worked on further until they can move freely, or they are replaced with other probes 36. Probes 36 that cannot move freely are unable to buckle and flex when normally when placed in contact with the semiconductor chip 70, thereby preventing assembly 20 from functioning normally. Once all probes 36 move freely, template alignment tool 72 is moved back into the position where micro-holes 32 and 34 are in relative alignment using gauge pins 80 inserted in holes 54. Template alignment tool 72 is then re-clamped to lower die 28. Template 26 is then attached to lower die 28, such as by use of adhesive strips 38, which are typically KAPTON tape. Other methods of attaching template 26 to lower die 28 include ultrasonic welding, adhesive glue, mechanical fasteners, or other conventional means of attachment known to the art. Template 26 is then separated from template alignment tool 72.
Upper die 22 is then attached to spacer 24 and lower die 28 in step 116. Upper die 22 is carefully lowered and moved into position so that heads 56 of probes 36 extend up through micro-holes 30 formed in upper die 22. In step 118, gauge pins 80 are inserted through holes 54 in upper die 22, spacer 26, and lower die 28 in order to place upper die 22, spacer 26, and lower die 28 in relative alignment. Screws 48 then join upper die 22 to spacer 26 and lower die 28 creating assembled probe assembly 20.
Contact heads 60/61 of probes 36 are then planarized in step 120 so that contact heads 60/61 are on a generally uniform plane for making contact with the footprint of semiconductor chips 70. Collectively, all of contact heads 60/61 are referred to as the probe head. Lapping fixtures are one exemplary method of planarizing contact heads 60/61. In step 122, the probe head is cleaned. An ultrasonic clean process with isopropyl alcohol is one exemplary method of cleaning contact heads 60/61. The assembly process is then complete and ends in step 124.
The use of template 26 and boss 40 presents numerous advantages. Having the ability to attach template 26 to boss 40 with adhesive strips 38, for example, eliminates the need to use glue in assembling probe assembly 20. Glue is commonly used to hold probes 36 in position within lower die 22 during attachment of upper die 22. The use of solvent to clean probe assembly 20 does not always result in the freeing all glued probes, consequently requiring a rebuild or scrapping of those assemblies 20. The use of template 26 eliminates these costly and problematic glue steps and allows for a more reliable and cost effective assembly process.
The use of template 26 also helps to secure probes 36 during the assembly process. Without template 26, probes 36 can slip out of lower die 28 more easily, requiring reassembly of probe assembly 20, thereby adding cost to the device. However, template 26 helps to hold probes 36 in position during attachment of upper die 22, thereby making for a more reliable manufacturing process. The use of the template 26 and boss 40 also facilitates repair. Together, template 26 and boss 40 enable the repair and replacement of a damaged probe 36 without the entire disassembly and reassembly as discussed later in more detail.
The use of template 26 also simplifies the attachment of upper die 22 to the remainder of probe assembly 20. Template 26 places probes 36 in alignment with micro-holes 30 formed in upper die 30. Consequently, placing upper die 22 onto probe assembly 20 becomes simple and straightforward with holes 30 and 32 in alignment.
Template 26 is held down against boss 40 with, for example, adhesive strips 38. Heads 56 of probes 36 protrude up through the top surface of template 26. Boss 40 resides in the opening 44 of spacer 24. Screws 48 secure spacer 24 to lower die 28.
Although the present invention has been described in detail, it will be apparent to those of skill in the art that the invention may be embodied in a variety of specific forms and that various changes, substitutions, and alterations can be made without departing from the spirit and scope of the invention. The described embodiments are only illustrative and not restrictive and the scope of the invention is, therefore, indicated by the following claims.
